CSC 600 Internetworking with TCP/IP Unit 2: Client-Server Model and Sockets(Ch.2, 3, 21, 22) Dr. Cheer-Sun Yang Fall 2000

Theoretical Basis for Data Transmission Information can be transmitted through a medium by varying some physical property. The physics of the universe (noise, distortion, attenuation) places some limits on what can be sent over a channel. Purpose of physical layer – to transport a raw bit stream from one machine to another.

How do we transmit these waves? Feed an electrical signal to the antenna of a transmitter The signal makes the atoms of the antenna vibrate (changing energy levels). This change causes the antenna to emit electromagnetic waves.

Bits can be sent as a voltage or current through a wire For example: zero = +1 volts one = -1 volts (Later we’ll see some problems with this representation…) Sample Data Representation voltage

Bandwidth A given transmission medium can accommodate signals within a given frequency range. The bandwidth is equal to the difference between the highest and the lowest frequencies that may be transmitted. For example, a telephone signal can handle frequencies between 300 Hz and 3300 Hz, giving it a bandwidth of 3000 Hz. This means, very high- or low-pitched sound cannot pass through the telephone system. Sometimes, bandwidth is used to denote the number of bits that can be transmitted.

Chapter 2 Transmission Media Copper Wires Glass Fibers Radio Satellites Geosynchronous Satellites Low Orbit Satellites Low Orbit Satellite Arrays Microwaves Infrared Laser Lights

Twisted Pair Insulated Copper wires, about 1mm thick Twisted, to avoid forming an antenna: reduces interference Two major kinds –Cat 3 (1988 and earlier) four pairs: (allows four telephone lines) –Cat 5: (new installations) more twists per centimeter, and Teflon insulation more suitable for high speed networks. Shielded vs. Unshielded: –shielded twisted pair (STP) (shield serves as ground, some applications in business use this, but becoming more rare) –unshielded twisted pair (telco local loop to home is usually UTP)

COAXIAL CABLE

More about Coaxial Cable Coaxial cable typically transmits information in one of two modes: baseband or broadband mode. Baseband mode - the cable’s bandwidth is devoted to a single stream of data. Broadband mode - the bandwidth is divided into ranges. Each range typically carries separate coded information.

Ethernet Transceiver: used to connect an Ethernet and a host. host transceiver

Ethernet

10/100 Ethernet 10 BASE T 10 Mbps basebandTwisted-pair

Chapter 3 Internet Architecture Repeater – extending a cable (UTP, STP, Optical Fiber). Bridge – interconnecting two LANs which operate in the data link layer. Example: 2 TCP/IP LANs. Router – interconnecting two WANs. e.g., the Internet, which operate on the network layer. Example: an Ethernet LAN and a Token Ring. Gateway – interconnecting different networks in the application layer. For example, one network implements OSI and another implements TCP/IP.

Router

Interconnect Through IP Routers In a TCP/IP internet, special computers called IP routers or IP gateways provide Interconnections among physical network. Routers use the destination network, not the destination computer, when forwarding a packet.

What is a Network? In a TCP/IP internet, TCP/IP can be built over various media-access sublayer. If TCP/IP over Ethernet, it is the Internet architecture. If TCP/IP over ring connection, it is the IBM Token Ring. But, all are called TCP/IP networks from the router’s point-of-view. Thus, IP datagrams can be passed from Internet to Token Ring without going through application level gateway. See Fig. 3.3

A Virtual Network

Unanswered Questions How does a router identify a computer?  Chapters 4, 5, 6 cover this. How does an IP datagram look like?  Chapter 7 introduce this. How does a TCP segment look like?  Chapter 13 talks about this. How does an application use the services provided by TCP or UDP?  next topic. (Ch. 22)

Chapter 21 Client-Server Model A host will execute the server program which will always wait for requests for connection. Another host will execute a client program. The client and the server will use 3-way handshaking to initiate the communication. After the connection is established, the client and the sender can send/receive messages. This model is called a connection-oriented model.

Client-Server Model(cont’d) The previous model uses TCP/IP. If UDP/IP is used, after the connection is called, the client only receives the remote endpoint information. A client then uses the information to identify the destination address for a socket. This model is called a connectionless model.

Chapter 22. Socket Interface Services provided by TCP and UDP to the application layer, called an Application Program Interface (API). There are many versions on various machines: BSD UNIX, Windows, Linux. TCP provides a connection-oriented service; UDP provides a connectionless service.

Socket Interface The network interface is similar to other I/O system calls: OPEN, READ, WRITE, CLOSE. In addition, it also provides the following: –Create a socket –Bind – specifying a local address –Connect – connecting sockets to a destination -Listen – a server waits for a connect request -Accept – a server accepts the request -Many others.

Socket Address Structure struct sockaddr_in { u_char sin_len; u_short sin_family; u_short sin_port; struct in_addr sin_addr; char sin_zero[8]; };

Internal Socket Structure struct { int family; int type; int protocol, struct sockaddr_in local_socket_addr; struct sockaddr remote_socket_addr; };

Socket System Calls Int socket (int Family, int Type, int Protocol); Family – defines the protocol group: IPv4, IPv6, UNIX domain Type – stream socket, datagram socket, or raw socket Protocol – usually is set to zero for TCP and UDP

Connection-Oriented socket() bind() listen() accept() read() write close() read() write() connect() socket() Blocks until server receives a connect request from client connect request data server client